1. Field of the Invention
The present invention relates to an excess current interrupting structure which shuts off an electric current path connected to a load with high responsiveness and high reliability when the current path receives a current in excess of a predetermined value (referred to hereinafter as an excess current), and more particularly to an excess current interrupting structure which is formed by using a conductive wire which generates heat under the excess current. The wire is made of a good conductor, such as gold, copper and aluminum, has a relatively high resistance in the wire form, is covered with a resin and is placed in a current path such that the path is completely shut off once the excess current flows through the path.
2. Description of the Related Art
Prior art devices employ fuses, which are inserted in a current path connected to a load, and melt away under an excess current to shut off the current path. As well known in the art, a cartridge fuse 101 of the structure shown in FIG. 16 is used when a target excess current is high, as in the case of a current path connected to a load, and a tubular fuse 102 of the structure shown in FIG. 17 is used with electronic devices of small target excess current. However, these fuses have the following problems:
(1) The cartridge fuse 101 has a complicated construction, a high degree of difficulty in manufacturing and a high price, so that it causes a large cost increase in the overload protection of a current path connected to the load.
(2) Both the cartridge fuse 101 and the tubular cartridge 102 use a fuse holder, so that when they are placed in an electronic device, they occupy a large space.
The new excess current interrupting structure of this invention uses a conductive wire which generates heat when an excess current flows through it. The wire is made of a good conductor, such as gold, copper or aluminum, has a relatively high resistance in the wire form, is covered with a resin, and is placed in a current path such that the path is completely shut off once the excess current flows through the path. This enables both securing of the capacity of an electric current path (heat radiation to the surrounding of the path) and fixing of a pair of electric conductors (for example, terminals) connected to the conductive wire. In addition, this structure is advantageous in view of its ease of handling, resistance to the environment and insulating characteristics with respect to peripheral parts.
However, in the case of a prior art structure in which the circumference of a conductive wire is coated with a resin, it should be noted that the carbonization of the resin around the conductive wire occurs when the conductive wire is melted under an excess current. This carbonized portion forms a bypass electric current path (carbonized path). Thus, even when the conductive wire is melted by the excess current, the bypass electric current path formed by the carbonized portion remains, and the excess current continues to be supplied to the load. Therefore, the prior art structures in which the circumference of the conductive wire is coated with a resin are impractical.
The present invention aims at providing a practical excess current interrupting structure in which the circumference of a conductive wire is coated with resin and shuts off with high responsiveness and high reliability an electric current path in which an excess current flows.
This invention is directed to providing an excess current interrupting structure comprising a conductive wire which generates heat under an excess current and connects a pair of electric conductors. The circumference of the conductive wire is covered by a thin resin layer formed in the vicinity of the path of the conductive wire.
In the invention, when the conductive wire connecting a pair of electric conductors generates heat under an excess current, causing the temperature thereof to increase, with the resin layer also generating heat at the same time to cause the temperature thereof to increase, the temperature rise in the resin layer is concentrated in the section over which the thin coated portion is formed. As a result, thermal stress and a large quantity of gas form upon the carbonization of the resin in the section of the thin resin layer, rather than in the section which has a thick resin coating. In other words, the rate of increase in thermal stress and gas pressure in the section of the thin resin layer is sharper than the increase in the thermal stress and the pressure in the section of the resin layer which is thicker.
Therefore, when a carbonized path is formed because of the carbonization of the resin around the conductive wire under an excess current, the thermal stress and gas pressure reach a critical bursting pressure in the section of the thinner resin coating and burst the section on a small scale. In the meantime, the thermal stress and gas pressure remain well short of the bursting pressure of the section of the thicker resin coating. Consequently, the section of thin resin layer is instantaneously destroyed, together with the carbonized path. Typically, a crack forms in the section of the thin resin layer when the bursting takes place, and it does not matter whether the conductive wire is melted by the excess current. As a result, when a load current flowing in the current path between a pair of electric conductors reaches an excess current level, the current path in the section of thin resin layer is shut off with high responsiveness and high reliability while the section of thicker resin coating remains undestroyed. In addition, the thickness of coating of the section of thin resin layer may be regulated so that the shut-off time for the current path is adjusted accordingly.
The conductive wire of this invention comprises a thin metal wire made of a conductive metal of a high electric conductivity. This enables flexibility in placement of the conductive wire in a device.
Furthermore, the excess current interrupting structure of this invention may have a plurality of conductive wires connecting the pair of electric conductors. This enables the expansion of the heat generation region under the excess current flow between the pair of electric conductors, thus expanding the destruction starting region. In this configuration, the diameter of the electric wire changes inversely in proportion to the number of the wires.
Still further, in this invention, the material and volume of the conductors are selected so that the generation of heat under the excess current concentrates at the conductive wire. This enables heat generation with high responsiveness when the excess current flows in the current path.
Furthermore, in this invention, the conductors are of a shape suitable for mounting on a circuit board so that standard surface mounting techniques are used in accordance with standard surface mounting specifications. It is also possible that the thinner resin coating is formed by transfer molding using a metal mold, resulting in a much simpler production process. The excess current interrupting structure of this invention may also include a sensor for detecting a change in a condition of the device. Such a sensor may be made of temperature sensing material and a physical force sensing material. Such a sensor may change its color under heat or stress so that the change is recognized even when the destruction of the excess current interrupting structure is not clearly visible.